TS copy of letter to W H Preece from Marconi. Refers to lecture to be gven by Preece [in July to the Post Office] in London on the subject of wireless telegraphy. Offers the use of his [Marconi's] equipment to demonstrate signalling across the room. Asks Preece not to explain Marconi's methods as Marconi wishes to wait until he can present all his findings to a scientific society. Marconi apologises in advance for the condition of his apparatus, as his best set is not available, and asks to set the apparatus up himself.